1. Field of the Invention
The present invention relates to a distillation column used for carrying out a distillation operation widely employed in a large number of industrial processes, and more particularly, relates to a mechanical-heat-pump distillation column which is a distillation column that includes a mechanical heat pump system.
2. Description of the Related Art
A section located above a feed stage of a distillation column used for carrying out continuous distillation is referred to as a rectifying section, and a section below the feed stage is referred to as a stripping section. The distillation column is provided with an overhead condenser for cooling and condensing an overhead vapor, and a reboiler for heating and boiling a column bottom liquid.
A mechanical-heat-pump distillation column is known as a distillation column improved in view of saving energy. As the mechanical-heat-pump distillation column, a vapor re-compression system (VRC) is known as described in the pamphlet “Sulzer Chemtech Distillation and Heat-Pump Technology, Production of 1-Butene from Tail Gas of a MTBE Plant” from Sulzer Chemtech Ltd., and U.S. Pat. No. 4,718,986 A. Also, a heat integrated distillation column (HIDiC) is known as described in JP H08-66601 A, JP 2004-16928 A, WO 2011/043199, and JP 2013-208561 A.
In the VRC, an overhead vapor withdrawn from the column top is compressed by a compressor to be increased in temperature, and the temperature-increased fluid is used as a heat source of the reboiler of the VRC. It can be said that, in the VRC, heat of the column top of one column can be fed to the column bottom of the same column by using a mechanical heat pump system. Therefore, the amount of heat to be supplied at the reboiler can be reduced, and energy consumption in the distillation column can be reduced. The VRC is also referred to as a mechanical vapor recompression (MVR).
The HIDiC has a basic configuration in which the rectifying section and the stripping section of one distillation column are separated from each other. The operating pressure of the rectifying section is set to be higher than the operating pressure of the stripping section in order that the operating temperature of the rectifying section becomes higher than the operating temperature of the stripping section. To this end, a high-pressure column including the rectifying section and a low-pressure column including the stripping section are used. A vapor withdrawn from the column top of the low-pressure column is compressed by a compressor to be increased in temperature, and then transferred to the column bottom of the high-pressure column. Also, a liquid withdrawn from the column bottom of the high-pressure column is transferred to the column top of the low-pressure column. Further, the HIDiC is configured so that heat can be exchanged between the rectifying section and the stripping section. Accordingly, heat is transferred from the rectifying section to the stripping section (internal heat exchange), and therefore, it is possible to reduce the amount of heat to be supplied at the reboiler and to reduce the amount of heat to be removed at the overhead condenser. In other words, by transferring heat from the rectifying section to the stripping section by the heat exchange, the heat to be supplied at the reboiler and the heat to be removed at the condenser can be at least partially substituted with the transferred heat, and as a result, a distillation apparatus that has extremely high energy efficiency can be obtained.
In the mechanical-heat-pump distillation column, a vapor withdrawn from any position of the distillation column is compressed by the compressor to be increased in temperature. This makes it possible to perform heat exchange for feeding heat of an overhead vapor to the reboiler, or to perform heat exchange for transferring heat from the rectifying section (the high-pressure column) to the stripping section (the low-pressure column).